Perhaps you should try some spelling variations:
"ground penetration radar" » "grout penetration radar", "cloud penetration radar", "rod penetration radar", "ground penetrating radar", "ground penetration rate", "ground penetration der"
"ground penetration raman" » "grout penetration raman", "cloud penetration raman", "rod penetration raman", "ground penetration human", "ground penetration rate", "ground penetration bmn"
"ground penetrating radars" » "ground penetrating radar", "ground penetrating bars", "ground penetrating beams"

Try widening your search to All Fields.

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (((("ground penetration radar") OR ("ground penetrating radars"))) OR ("ground penetration raman")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"ground penetration radar\") OR (\"ground penetrating radars\"))) or (\"ground penetration raman\"))".
• Removing quotes may allow a broader search: (((( ground penetration radar ) OR ( ground penetrating radars ))) or ( ground penetration raman )).
• Adding a wildcard symbol may retrieve word variants: (((("ground penetration radar") OR ("ground penetrating radars"))) or ("ground penetration raman"))*.